1. Field of the Invention
The present invention relates to an electronic endoscope including a flexible conduit, and a video-signal processor to which the flexible conduit is detachably joined.
2. Description of the Related Art
The flexible conduit of the electronic endoscope has a solid image sensor, such as a CCD (charge-coupled device) image sensor, provided at the distal end thereof, and an objective lens system associated therewith. An object to be photographed is focused, as an optical image, on a light receiving surface of the CCD image sensor by the objective lens system, and the optical image is converted into analog electric image-pixel signals by the CCD image sensor.
The flexible conduit includes an optical guide extending therethrough, and the optical guide terminates at a light-emitting end face at the distal end of the flexible conduit. On the other hand, the video-signal processor of the electronic endoscope also includes an optical guide provided therein. When the flexible conduit is joined to the video-signal processor, one end of the optical guide of the video-signal processor is connected to a proximal end of the optical guide of the flexible conduit. The video-signal processor further includes a light source, and a collective lens system associated therewith, and light rays emitted from the light source are focused on the other end face of the optical guide of the video-signal processor by the collective lens system.
Thus, a front area of the distal end of the flexible conduit is illuminated by the light rays emitted from the light-emitted end face or distal end face of the optical guide of the flexible conduit.
For reproduction of a photographed image as a color image, for example, an RGB-field-sequential-type color-imaging system is introduced in the electronic endoscope. Namely, a rotary RGB color-filter is intervened between the light source and the inner end face of the optical guide of the video-signal processor, and the RGB color filter is rotated at a given frequency of rotation, whereby an object to be photographed is sequentially illuminated by red light rays, green light rays, and blue light rays. Thus, a red optical image, a green optical image, and a blue optical image are sequentially focused on the light receiving surface of the CCD image sensor at given time intervals.
When either a red, green, or blue optical image is focused on the light receiving surface of the CCD image sensor, the optical image is converted into analog image-pixel signals by the CCD image sensor. The analog electric image-pixel signals are then read out of the CCD image sensor, in succession, by a CCD driver circuit. The read analog image-pixel signals are fed to the video-signal processor, in which the analog image-pixel signals are subjected to suitable image-processings. Then, the processed analog image-pixel signals are converted into digital image-pixel signals by an analog-to-digital (A/D) converter, and are temporarily stored in one of three frame memories provided for the digital red image-pixel signals, digital green image-pixel signals, and digital blue image-pixel signals.
The digital red, green, and blue image-pixel signals are read from the frame memories, and are outputted to a digital-to-analog converter (D/A), in which these digital image-pixel signals are converted into an analog color video-signal. The converted analog color video-signal is passed through a low-pass filter, and is amplified by an amplifier. Then, the amplified analog color video-signal is fed to a TV monitor for reproduction of the photographed optical image.
As is well known, the video-signal processor of the electronic endoscope is provided with a timing generator for outputting several series of clock pulses, having a given identical frequencies, to the A/D converter, the frame memories and so on. Namely, the conversion of the analog image-pixel signals into the digital image-pixel signals by the A/D converter, the storage of the digital image-pixel signals in the frame memories, and the outputting of the digital image-pixel signals from the frame memories to the D/A converter are carried out on the basis that the series of clock pulses have the same frequency, as the aforementioned operations.
In the electronic endoscope, as mentioned above, the flexible conduit is exchangeable for another flexible conduit. This is the reason why the flexible conduit is detachably joined to the video-signal processor. In general, flexible conduits to be joined to the video-signal processor are classified into two groups: one group is represented by, for example, a flexible conduit for a bronchoscope; and the other group is represented by, for example, a flexible conduit for a photogastroscope. The CCD image sensor incorporated in the flexible conduit for the bronchoscope is smaller than that of the flexible conduit for the photogastroscope.
Furthermore, if the NTSC (National Television System Committee) system is introduced in the electronic endoscope, analog image-pixel signals obtained from the CCD image sensor for the bronchoscope, must be converted into digital image-pixel signals by the A/D converter on the basis of a series of clock pulses having a frequency of 12.2727 MHz. Also, analog image-pixel signals obtained from the CCD image sensor for the photogastroscope, must be converted into digital image-pixel signals by the A/D converter on the basis of a series of clock pulses having a frequency of 14.3182 MHz.
On the other hand, if the PAL (Phase Alternation by Line) system is introduced in the electronic endoscope, analog image-pixel signals obtained from the CCD image sensor for the bronchoscope, must be converted into digital image-pixel signals by the A/D converter on the basis of a series of clock pulses having a frequency of 14.75 MHz. In conjunction with this, analog image-pixel signals obtained from the CCD image sensor for the photogastroscope, must be converted into digital image-pixel signals by the A/D converter on the basis of a series of clock pulses having a frequency of 17.0625 MHz.
Accordingly, in the video-signal processor of the conventional electronic endoscope, the timing generator is able to selectively output at least two kinds of clock pulses having frequencies of 12.2727 MHz (14.75 MHz) and 14.3182 MHz (17.0625 MHz).
Recently, with the spread of video cameras, video tape recorders, video-image processing computers and so on, a standardization for the processing of digital video signals has been proposed and put into practice. For example, in Rec. 601 Standardization (Recommendation ITU-R BT.601), a component-type digital color video signal, composed of a luminance signal component and two kinds of color-difference signal components, is recommended as the digital color video signal to be used and, further, this component-type digital color video signal should be processed on the basis of a series of clock pulses having a frequency of 13.5 MHz.
On the other hand, there is a demand for connecting the electronic endoscope to peripheral equipment other than a T.V. monitor, such as a printer, a video tape recorder, an image-processing computer and so on. Accordingly, it is necessary to modify the electronic endoscope in accordance with Rec. 601 Recommendation before the demand can be satisfied.
For the modification as mentioned above, it is easily conceivable for those skilled in the art to incorporate an analog color-matrix circuit and another analog-to-digital (A/D) converter in the video-signal processor of the electronic endoscope.
In particular, firstly, the analog color video-signal outputted from the D/A converter is inputted to the analog color-matrix circuit, where the analog color video signal is converted into a luminance signal and two kinds of color-difference signals. Then, the analog luminance signal and the two kinds of analog color-difference signals are inputted to the other A/D converter, in which the analog luminance signal and the two kinds of analog color-difference signals are converted into a digital luminance signal and two kinds of digital color-difference signals on the basis of clock pulses having the frequency of 13.5 MHz.
Thus, the electronic endoscope can output the digital component-type digital color video signal, composed of the digital luminance signal and the two kinds of digital color-difference signals, which can then be processed on the basis of the series of clock pulses having the frequency of 13.5 MHz.
Nevertheless, this approach is unadvisable because a quality of a reproduced color image may be deteriorated due to the fact that the analog color video signal is subjected to the analog color-matrix conversion-processing. Also, since a low-pass filter and an amplifier must be provided at both the input and output sides of the analog color-matrix circuit, respectively, an overall arrangement of the control circuit board of the video-signal processor becomes bulky and complicated.